Most people are familiar with desiccant from the small packets we find enclosed with a new pair of shoes, in a bag of beef jerky, or in some medication bottles. These packets almost always say “Do Not Eat,” and I get that for the ones in the beef jerky or the pill bottles, but I just don’t understand why they put it on the desiccant packets bound for a shoe box…
Anyway, desiccant (in MUCH larger volumes than the household examples above) are also used to get water vapor out of compressed air. Desiccant dryers are popular because they’re effective and reliable. The most common design consists of two vertical tanks, or towers, filled with desiccant media – usually activated alumina or silica gel.
These materials are prone to adsorption (similar to absorption, only it’s a physical process instead of a chemical one) which means they’re good at trapping, and holding, water. In operation, one of these towers has air coming in it straight from the compressor (after it’s become pressurized, remember, it still has just as much water vapor in it as it did when it was drawn in…up to 5% of the total gas volume.)
When that tower’s desiccant has adsorbed water vapor for long enough (it’s usually controlled by a timer,) the dryer controls will port the air through the other tower, and commence a restoration cycle on the first tower. So, one is always working, and the other is always getting ready for work.
There are three methods by which the desiccant media can be restored:
- Regenerative Desiccant Dryers send a purge flow of dry air (fresh from the operating tower’s discharge) through the off-line tower’s desiccant bed. This dry air flow reverses the adsorption process, and carries the water away as it’s exhausted from the dryer. This is simple and effective, but it DOES use a certain amount of your compressed air.
- Heat Of Compression Desiccant Dryers use the heat from pressurized air straight from the compressor(s). This hot air is directed through one tower, where it removes moisture from the desiccant. It then flows through a heat exchanger where it’s cooled, condensing the moisture, before it flows through the other tower to remove any remaining moisture. This method doesn’t add to your compressed air usage, but it only works with oil-free compressors.
- The third method uses a hot air blower to flow heated air through the off-line desiccant bed. It’s similar to the Regenerative type, but it doesn’t use compressed air. However, they DO require a certain amount of wattage for the heater…remember, electricity isn’t cheap either.
As an EXAIR Application Engineer, it’s my job to help you get the most out of our products, and your compressed air system. If you have questions about compressed air, call me.